1. Field of the Invention
This invention relates to a developing apparatus for photocopiers, printers, facsimile machines, and the like which uses one of an electrophotographic method, an electrostatic recording method, and the like.
2. Description of Related Art
FIG. 9 is a cross section showing a schematic structure of a conventional image forming section. In the image forming section such as a photocopier shown in FIG. 9, a photosensitive body 101 as an image carrier is charged with a charging apparatus 102, and an electrostatic latent image is formed on a circumferential surface of the photosensitive body 101 by exposing images of the original document at an exposure position 103 with an exposure optical system. The electrostatic latent image formed on a circumferential surface of the photosensitive body is developed by a developing apparatus 109 to form a toner image, and the toner image is transferred onto a transfer material in application of a voltage at a transfer apparatus 104. After the photosensitive body 101 to which the transfer is made is cleaned by a cleaning apparatus 106, remaining charges are removed upon exposure before charging, and the above steps are repeated to form images.
The developing apparatus 109 is disposed with a developer carrier 111 (hereinafter referred to as “developing sleeve”) arranged with a certain gap with respect to the photosensitive body 101. A means for restricting the gap is defined by the difference between the outer diameter of a restricting member 118 (striking roller) rotating around the same shaft of the developing sleeve 111 and the outer diameter of the developing sleeve 111, and generally adapts the striking roller method guaranteeing the gap by pushing the developing sleeve 111 toward the photosensitive body 101.
According to a recent tendency of higher copying and printing speeds, there is a developing apparatus having two developing sleeves corresponding to a high speed. FIG. 10 is a schematic cross section showing a developing apparatus of a conventional example having two developing sleeves. With this developing apparatus, a developing sleeve 111a (hereinafter referred to as “developing upper sleeve”) as a first developing member located on an upstream side in an image forming process and a developing sleeve 111b (hereinafter referred to as “developing lower sleeve”) as a second developing member located on a downstream side are rotatively arranged at respective shafts of the developing apparatus.
The photosensitive body 101 may be subject to vibrations occurring during rotation due to size accuracy, and the rotation of the photosensitive body 101 may be disturbed if the developing apparatus 109 is secured under a state that the striking rollers 121a, 121b are in contact with the surface of the photosensitive body 101. The apparatus is structured in which elastic force of an elastic member such as a spring or the like absorbs the vibrations of the photosensitive body 101 by pushing a developing container 110 of the developing apparatus 109 toward the photosensitive body 101 via the elastic member 131 to absorb the vibrations of the photosensitive body 101.
FIG. 11 is a schematic cross section showing a developing apparatus of a second conventional example having two developing sleeves. In this conventional example, an apparatus is devised in which the developing upper sleeve 111a is rotatively supported to the developing container 110, in which the developing lower sleeve 111b is rotatively supported via a pair of independent rocking members 141, 141 whose rocking center coincides to the shaft supporting the developing upper sleeve on the opposite sides, and in which the rocking members are pushed to the photosensitive body 101 by an elastic member 151.
In the conventional developing apparatus having a sole developing sleeve, the gap between the developing sleeve 111 and the photosensitive body 101 is arranged with high accuracy in a limited range from a variety of viewpoints. This is substantially the same as the developing apparatus 109 capable high speed having two developing sleeve 111, and guaranteeing of gaps with high accuracy is needed at the gap between the developing upper sleeve 111a and the photosensitive body, the gap between the developing lower sleeve 111b and the photosensitive body 101, and the gap between the developing upper sleeve 111a and the developing lower sleeve 111b. 
The first conventional example in which plural developing sleeves 111 are secured between shafts in the same developing container, raises the following problems.
When the striking rollers 121a, 121b on the same shaft of the developing sleeve come in contact with the photosensitive body 101, the striking portions at four or more locations in total secured between the shafts located at front and rear positions contact to the same circumferential surface of the photosensitive body, and therefore, if pressing force is weak, at least one striking roller comes to have a gap with the photosensitive body 101. That is, the gap between the photosensitive body 101 and the developing sleeve 111 becomes larger than the desired amount, so that image defects such as scattering, blooming and the like may occur.
Conversely, if the pressing force is high, the developing container or the developing sleeve 111 may be subject to stressing, and it is impossible to guarantee main gaps. Moreover, this may disturb rotation of the photosensitive body and the developing sleeve 111, and also may grind the striking rollers 121a, 121b during the use. Because the gap between the photosensitive body 101 and the developing sleeve 111 decreases more than the desired amount, image defects such as blurring may occur.
In the second conventional example in which another developing sleeve 111b is rotatively supported via the rocking members 141 and in which the rocking members 141 are pushed to the photosensitive body 101 by the elastic member 151, there raise the following problems.
The gap between the developing sleeve 111a secured to the developing apparatus 109 and the photosensitive body 101, as well as the gap between the another developing sleeve 111b supported via the rocking members 141 and the photosensitive body 101 are guaranteed, and the axial centers at the ends of the respective developing sleeves 111a, 111b are guaranteed.
However, fundamental lines of the respective developing sleeves 111a, 111b may be shifted since the photosensitive body 101 is hit at the four points in the same way as in the first conventional example in which the developing container is secured, as described above, or namely, the gap between the developing upper sleeve 111a and the developing lower sleeve 111b becomes uneven in the axial longitudinal direction, so that unevenness in toner coating formed on the respective developing sleeves 111a, 111b may cause image defects.
Furthermore, in this conventional example, a magnetic sealing member is secured to the developing container to prevent the toner from leaking out of the end of the developing sleeve 111, and the gap between the developing sleeve 111b on the side supported at the shaft thereof via the rocking member 141 and the magnetic sealing member may vary depending on the vibrations of the photosensitive body 101. That is, there raises a problem that the developing sleeve rotation load torque increases due to toner leakage from the widen gap or due to toner adherence from the reduced gap.